This is the simplest of the three common tie knots. It is slightly asymmetrical and has a casual look. It is great for narrow collars and wide ties.
jeffreyf posted an instructable for how to tie a Four in Hand and his own knot, but didn't cover the Half Windsor or Full Windsor. Since all of these knots use many of the same moves, I'm including a Four in Hand in this series.
The pattern to remember is "Over, Under, Around, and Through".
Step 1: Ready?
Lift your collar and place the tie around your neck with the front side facing out. The wider end of the tie should be on your right.
Step 2: Over
Grab the wide end of the tie and cross it over the narrow tail.
Step 3: Under
Bring the wide end back under the narrow tail. Then grab the narrow tail just under the newly-formed loop.
Step 4: Around
Loop the wide end around the front of your left hand.
Step 5: Through
Bring the wide end of the tie through the Y at the top of the knot and also through the loop you just formed around your hand.
Step 6: Tighten
Tighten the knot by grabbing the tail and holding under the knot while you pull down on the wide end.
Then bring the knot up to the collar by pulling up on the knot while you pull down on the narrow tail.
Repeat these two motions as necessary.
Step 7: Finish
Flip down your collar and adjust the placement of the knot.
Step 8: Dress Down
The Four in Hand is an asymmetrical knot: one side of the knot is higher than the other, so there's a diagonal line at the top of the knot. This makes it great for the "I had a long day at work, but I'm winding down now" look.
Loosen the knot. Undo a button or two. Have a beer.